dart2js: use Es6 maps when available.

pkg/compiler/lib/src/js_backend/namer.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_backend;
 
 /**
  * Assigns JavaScript identifiers to Dart variables, class-names and members.
  *
  * Names are generated through three stages:
@@ skipping 227 matching lines @@
     "MouseEvent", "Navigator", "Node", "NodeList", "Option", "Plugin",
     "ProcessingInstruction", "Range", "RangeException", "Screen", "Select",
     "Table", "TableCell", "TableRow", "TableSelection", "Text", "TextArea",
     "UIEvent", "Window", "XMLHttpRequest", "XMLSerializer",
     "XPathException", "XPathResult", "XSLTProcessor",
 
     // These keywords trigger the loading of the java-plugin. For the
     // next-generation plugin, this results in starting a new Java process.
     "java", "Packages", "netscape", "sun", "JavaObject", "JavaClass",
     "JavaArray", "JavaMember",
+
+    // ES6 collections.
+    "Map",
   ];
 
   static const List<String> reservedGlobalObjectNames = const <String>[
       "A",
       "B",
       "C", // Global object for *C*onstants.
       "D",
       "E",
       "F",
       "G",
@@ skipping 1501 matching lines @@
         if (!first) {
           sb.write('_');
         }
         sb.write('_');
         visit(parameter);
         first = true;
       }
     }
   }
 }